1. Field of the Invention
This invention relates in general to servo systems for disk drives, and relates more particularly to seek systems for use in such disk drives employing sector servo patterns.
2. Prior Art
In current disk drive systems, it is common to employ some type of servo system to position one or more heads at a desired one of a plurality of concentric recording tracks on a rotating recording disk surface and to maintain the head centered over the desired track during reading or writing of data in the desired track. Such servo systems operate in two modes, the first of which is called track seeking, in which a head actuator system moves the head or heads from the current track position to the desired or target track position. During this phase of the servo system operation, seek algorithms are employed to control the acceleration and deceleration of the actuator to optimize the time required for the head to reach the desired track. The optimum acceleration and deceleration values generated by the seek algorithm, as well as the seek time, will be a function of the "distance-to-go" or number of tracks to be crossed to reach the desired track.
In the second mode of operation, after the head reaches the target track, the servo system operates in a track following mode to maintain the head centered over the target track for reading or writing of data.
The time taken to move a head between tracks in a disk drive is one of the elements of what is known as the "access" time and is one of the most important performance characteristics of a disk drive. To minimize the access time for a drive of given mechanical configuration and actuator performance requires an access motion control system which will control the velocity of the head in time optimal fashion and which will bring the head accurately to rest on the desired track.
The access motion is, therefore, necessarily of wide bandwidth and the access control system is subject to the stability and error constraints of such systems. Conventionally, these wide band requirements have necessitated the use of a continuous position reference source such as a separate servo surface. In such a system, near time-optimal access motion has been accomplished by means of a derived continuous distance-to-go signal acting on a reference velocity curve generator which, via a high gain closed loop, forces the actual velocity of the head to follow a time-optimal reference velocity profile from the curve generator.
In a typical velocity servo loop type of seek algorithm, the actuator is accelerated in the direction of target track with maximum available acceleration during the acceleration phase as shown in FIG. 2A, until it attains the desired velocity for deceleration. This is called the switch point. Then the actuator is decelerated in a controlled fashion, as also shown in FIG. 2A, so that it comes to rest at the target track. At all times during deceleration, the velocity of the actuator is controlled close to a desired velocity, and a curve representing this velocity is called the desired velocity profile as shown in FIG. 2D. The desired velocity decreases with the distance to the target, reaching zero at the target track. Plots of actuator velocity and actuator position as functions of time are shown in FIGS. 2B and 2C, respectively.
This conventional approach is not available with a sector servo system, where position information is interspersed circumferentially on the disk surface with blocks of data, since direct head position and velocity information is available only at the periodically occurring servo sampling times. It is thus difficult to reconcile the use of sector servo in a disk drive with low access times.
Various access control schemes for sector servo disk drives have been proposed in the prior art. One of these, described in U.S. Pat. No. 4,103,314 "Motion Control System", is an access control system in which the actuator is energized to cause the head to follow a constant velocity portion of a desired velocity profile. The constant velocity is such that the passage of the head over track centers is synchronized with the timing of the servo sectors. The normal servo sector position error signal, as also generated during track following, may thus be used during the access motion to keep the velocity constant.
During brief initial acceleration and final deceleration stages of the motion in this prior art system, the full power supply voltage is applied to the actuator under open loop conditions. The motion of the head during an access does not approach time optimal motion since it is at constant velocity over all but a few tracks. The constant velocity is low, since the head traverses only one track per two sector periods and must be synchronized with the sector frequency. Furthermore, only in a low velocity system is it possible to effect the final deceleration under open loop conditions without significant final position error.
Another access control system for a sector servo disk drive is described in U.K. Pat. No. 1,527,950. This patent employs the so-called "bang-bang" technique of controlling head motion in which the maximum available power is used for both acceleration and deceleration. The system is switched between full forward power and full reverse power at a point which is calculated from the initial and target track addresses. The servo sectors are coded with track address information which is read by the head during the access motion and used to determine when the power is to be reversed. Although allowing the highest possible speeds to be attained during access motion, the described system does not employ any form of closed loop control during acceleration and deceleration. The position of the head when it comes to rest is thus unknown until a comparison can be made of the actual address of the track over which the head is most nearly situated with the target address. There is provision for a further shift of the head if the two addresses are not equal, but such shifts would add to the average access time.
Although most disk drives employing a sector servo system use a velocity servo loop for track seeking operation, many difficulties arise in its implementation. A schematic representation of one such prior art velocity servo loop is shown in FIG. 3A. Such a servo system is a type 1 servo system which has the property of not being able to follow varying input (velocity in the present case). A solution to this problem must be found, since during deceleration the actuator velocity must be maintained close to the desired velocity from the velocity profile.
To solve this problem, another acceleration component, called feed-forward, is added as shown in FIG. 3B. Typically, the value of feed-forward is calculated for ideal conditions and is not adapted for variations.
Another difficulty encountered in sector servo implementations is determining the proper acceleration-to-deceleration switch point. If the acceleration to control movement of the actuator is changed once per sample, then the change from acceleration to deceleration or switch point would also have a granularity of one sample time. If this decision can be made only after the current actuator velocity exceeds the desired velocity, the switch from acceleration to deceleration may take place up to one sample time later than the desired time. This error is significant for short seeks, especially those taking 10 samples or less in deceleration. This error can nearly be eliminated by making the desired time to switch fall on a sample time.